1. 邮寄;投寄
Mailing is the activity of sending things to people through the postal service.

2. 邮件
A mailing is something that is sent to people through the postal service.

1. the transmission of a letter

e.g. the postmark indicates the time of mailing

Synonym: posting

2. mail sent by a sender at one time

e.g. the candidate sent out three large mailings